The invention relates to a magnet coil for generating temporally rapidly varying magnetic fields, preferably in a nuclear magnetic resonance (=NMR) apparatus, which is divided into an even number of partial coils which are symmetrical with respect to one another, wherein each partial coil can be reproduced geometrically through one or two symmetrical operations, namely turning about an axis by 180.degree. and/or through reflection on a plane, on any other partial coil, and wherein each partial coil consists of several windings of a metallic material which are arranged on the surface of a geometric body.
Magnet coils of this type are known e.g. from U.S. Pat. Nos. 5,323,135 or 5,343,148.
An essential part of NMR systems which are generally used for tomography, partly also for spectroscopy, is a system of normally three gradient coils consisting of several partial coils which are supplied with currents of different strengths independently of one another. The purpose of these gradient coils is to superimpose spatially constant magnetic field gradients with adjustable strength to the uniform magnetic field BOz of the main field magnet of the NMR apparatus, the field being directed in the direction of a z-axis, where the direction of one of the gradients (dBz/dz) is usually parallel to the direction of the uniform main field BOz (z-gradient=axial gradient) and where the directions of the two other gradients (dBz/dx, dBz/dy) are orthogonal to each other and to the direction of the axial gradient consists, transverse to the direction of the main field (x- and y-gradients=transverse gradients). The spatial range, in which the magnetic field of these gradient coils is approximately linear, can be utilized for NMR methods with spatial resolution (imaging, space selective spectroscopy) if this range is not further limited by inhomogeneities of the main field.
The gradient coils may be embodied e.g. as x-, y- and z-coils on cylinder surfaces for conventional tomography magnets or as gradient coils for gradient accelerated NMR spectroscopy. In addition thereto, there are also known flat gradient plates for pole shoe magnets in NMR tomography. With respect to the geometric design of gradient coils, U.S. Pat. Nos. 5,666,054 and 5,563,567 are incorporated herein by reference, in which the spatial design of gradient coils is described in detail.
A particularly advantageous gradient coil system in which on the one hand, under presettable boundary conditions, the inductance L and additionally also technically relevant parameters of the magnetic coil arrangement, like e.g. the current density distributions, shielding effect etc. can be optimized independently of one another, is described in the initially cited U.S. Pat. No. 5,323,135.
A feature common to all the magnet coils mentioned herein consists in that they are formed of an even number of at least two partial coils which are arranged symmetrical with respect to one another in order to generate the normally desired spatially symmetric profile of the magnetic field. These partial coils can be transformed to one another basically by two symmetrical operations, namely the reflection by one plane or by turning about an axis by 180.degree.. The transverse gradient coils known e.g. from U.S. Pat. No. 5,323,135 consist of four partial coils of this type arranged on the surface of a circular cylinder; the transverse gradient coils known from U.S. Pat. No. 5,343,148 consist of two partial coils arranged on the surface of a circular cylinder. Axial gradient coils and coils for generating a homogeneous magnetic field in general consist of two partial coils arranged on the surface of a circular cylinder. The transverse gradient coils known from U.S. Pat. 5,959,454 consist of two coplanar partial coils.
The performance of nuclear magnetic resonance tomographs can, in general, be improved if the magnetic fields generated by the gradient coils are particularly strong. This is achieved, in addition to optimizing the geometric arrangement of the conductor paths of the gradient coils extending in windings, by operation at high electric currents. In order to keep the heat usually generated by Ohmic losses, as small as possible, the conductor paths in the gradient coils have a large cross-section and extend laterally to their direction on a surface such that the electric current density that causes the heating up remains as small as possible. A disadvantage of such a gradient coil arrangement is however that on rapid switching of gradients with such (streamline) gradient coils current distributions due to eddy currents occur in the broad conductor paths, disturbing the magnetic field of the gradient coil in the useful volume of the NMR-apparatus. Furthermore, the mentioned eddy currents may lead to an increased heating up of the gradient coil system as compared to operation with direct current and identical generated field strength, and thus to limitation of the performance data.
U.S. Pat. No. 5,998,998 describes a solution of this problem. U.S. Pat. No. 5,998,998 provides, instead of one broad conductor path, at least two conductor paths which are electrically connected in parallel and cross each other n times per winding, wherein n is an integral with n&lt;8, preferably n=1 or n=2 and where in the conductor paths extend essentially in a geometrically parallel manner between the crossings.
The division into parallel conductor paths with otherwise identical conditions has the effect that the number of conductor paths on the same surface is multiplied, at least doubled. Thus, the width of the individual conductor paths can be halved or even further reduced. This drastically reduces the possibility of generating eddy currents in the conductor paths, without changing the entire inductance L of the gradient coil.
The geometric interconnection between the individual, essentially in parallel extending, conductor paths by means of the current crossings guarantees that after switching there will be hardly any current redistributions in the conductor paths connected electrically in parallel due to different electric resistances or inductances.
However, magnet coils according to U.S. Pat. No. 5,998,998 have the disadvantage that the number of crossings equals at least half the number of windings in all partial coils and that the realization of the numerous crossings is relatively demanding such that the production of such coils is expensive.